AI controlled companions are very helpful throughout the game. They give you more firepower and make the game just a bit easier. Furthermore, they have special, and unique abilities for each of them. This makes finding all of them interesting because each of them will add a unique experience (in combat). In my opinion, the best companion is "Cheeseburger" as he just destroys the enemy force jumping from an enemy to another enemy.

Boomer (Dog, Land)
Found in Rae Rae's Pumpkin Farm in a cage behind the garage.

Nick Rye (Human, Air)
Found in Rye and Sons Aviation in Holland Valley.

Grace Armstrong (Human, Land)
Found in Lamb of God Church.

Cheeseburger (Bear, Land)
Found in a mission at F.A.N.G Center after taking it.

Hurk Drubman Junior (Human, Land)
Found in Fort Drubman.

Jess Black (Human, Land)
Found in Baron Lumber Mill.

Peaches (Mountain Lion, Land)
Found in Peaches Taxydermy.

Sharky Boshaw (Human, Land)
Found in Moonflower Trailer Park.

Adelaide Drubman (Human, Air)
Found in Drubman Marina.

Boomer - Easily found at one of the beginning missions. Easiest companion to get as you only have to take over a farm and set him free. He tags enemies and is good at stealth missions.

Nick Rye - To recruit him you'll require to complete one of the story line missions, and bring back his plane. After you bring his plane back, he'll agree to stay, and help you. He is a pilot of a plane which has gun turrets and it and drops bomb so he's great if you want to take out enemies from far away and not lose anything.

Grace Armstrong - You can recruit her after you help her defend a Church. Perfect at stealth missions ,and missions which require you to shoot long range. Her performance depends on what vantage point she has.

Cheeseburger - Found in a mission after seizing a outpost. He is a very useful companion, if your not planning to go stealthy. He can attract enemies attention and can still take out a lot of the enemies. Although, he will need your backup as he won't be able to kill everyone one his own.

Hurk Drubman Junior - Can be recruited after you complete one of the story line missions. He uses an RPG to destroy cars and planes. Not useful when your going to try and go stealthy. He can take out large groups of enemies.

Jess Black - You'll have to complete a story line mission with her. She can make you invisable to wild animals, making her a good companion for hunting. She can efficiently eliminate enemies with her bow with a lower chance of getting spotted.

Peaches - You will need to complete a story line mission with her. It will tag enemies for you and is good at being stealthy. She can sneak effectively and take out enemies silently without getting noticed. Although, she can easily be killed when fighting against a large group of enemies.

Sharky Boshaw - Can be recruited after completing a story line mission with him. His main weapon is a flamethrower which makes him a perfect companion to fight against large group of enemies. He is also able to destroy vehicles well. He also has an increased resistance against explosions and shock.

Adelaide Drubman - You will be able to recruit her after you offer to help her in a story line mission and after completing a story line mission with her. She flies a helicopter, which has a grappling hook place on it. The helicopter is also good at taking out large group of enemies, vehicles, and also helicopter.